1. Field of the Invention
The present invention relates to a processor for analyzing a tubular structure. More particularly, the present invention relates to a processor for analyzing a tubular structure which permits three-dimensional observation, in a diagnostically useful manner, of a tubular structure (system) such as blood vessels, the intestine, the windpipe, and the esophagus on the basis of three-dimensional imaging data of a patient captured by a diagnostic medical imaging apparatus (medical modality), and quantitative analysis of indices useful for diagnosis, including the thickness (including local changes such as stenoses and lumps) or the length of a tubular structure.
2. Description of the Related Art
It has been conventional practice in the diagnosis of blood vessels, to take X-ray images while administering an angiographic agent to a patient, in order to detect the presence of abnormalities on the basis of the image values.
Under such circumstances, the recent progress in imaging technology, such as X-ray CT (computer tomography) scanners and MRI (magnetic resonance imaging), has made it possible to easily obtain three-dimensional images of areas of blood vessels to be diagnosed in a patient. As a result, it has become possible to perform tomographic diagnosis of blood vessels using such three-dimensional images.
While X-ray angiographic tests require arterial injection of an angiographic agent, three-dimensional imaging of blood vessels using X-ray CT or MRI permits angiography of blood vessels by venous injection. Venous injection is less invasive and can alleviate the burden on patients.
Blood vessel imaging methods that do not substantially require angiographic agents have been studied. In particular, tests that do not require angiographic agents based on MRI, which is a non-invasive procedure permit minimization of the burden imposed on the patient even when the test is repeated over a short period of time.
There are merits and demerits depending on the dimension of the imaging used for diagnosis. Since X-ray imaging is two-dimensional, assessment of a topological abnormality based on this imaging is limited. For example, when using X-ray images taken in one direction, the diagnosis tends to underestimate the degree of blood vessel stenosis. Use of three-dimensional imaging permits observation of a three-dimensional form, thus improving the accuracy of diagnosis of the stenosis. Three-dimensional imaging is also effective for identifying the three-dimensional structure of the blood vessel or an aneurysm. X-ray angiographic imaging at present is inadequate at rendering capillaries. However, as the image quality of three-dimensional imaging improves in future, the scope of application of diagnosis using three-dimensional images will be expanded.
Under such circumstances, the following methods have been proposed for three-dimensional topography of blood vessels.
For example, Japanese Examined Patent Application Publication No. 3-10985 discloses a method for calculating the longitudinal vector of a tubular structure taken from three-dimensional imaging data derived from X-ray CT, MRI, and ultrasonic diagnostic apparatuses by means of a “vector detector”, calculating sections perpendicular to the tubular structure from the resultant longitudinal vector, and preparing and displaying images cut along these sections.
U.S. Pat. No. 5,891,030 discloses a method comprising the steps of extracting a center line of a tubular structure from an image of the tubular structure in a captured three-dimensional image; unbending the tubular structure along this center line in the longitudinal direction thereof into a stretched shape; displaying the stretched image; and displaying a volume-rendering image and a planar reformatted image corresponding to the former image.
Japanese Unexamined Patent Application Publication No. 2001-175847 discloses a method of producing MPR (Multiplanar Reconstruction) image data of a sectional surface perpendicular to the center line (center line, for example) of an extracted blood vessel, in sequence at positions along the center line of the blood vessel, and displaying the sequential images as an animation. This display method is intended to facilitate observation of the complex three-dimensional structure of blood vessels.
On the other hand, various techniques for extracting the center line of a blood vessel have already been proposed. For example, reference Onno Wink et al., “Fast delineation and visualization of vessels in 3-D angiographic images,” IEEE Trans. Med. Imag., vol. 19, no. 4, 337-346, 2000 presents a method comprising the steps of determining the center line of a blood vessel as a sequence of points represented by three-dimensional coordinates, and extracting contours of a blood vessel on sectional surfaces perpendicular to the center line at the individual points on this center line. This makes it possible to achieve three-dimensional extraction of the blood vessel center line and the contours of the blood vessel (blood vessel surface). The area or diameter of the blood vessel can be determined for the individual positions along the blood vessel center line by extracting the center line and the contours of the blood vessel. This paper also presents a graph with the blood vessel diameter as the ordinate and the distance along the blood vessel center line as the abscissa.
The stenosis ratio of a blood vessel is calculated using a reference blood vessel diameter A on the assumption of the absence of stenosis, and the actual diameter B of the stenosis site in accordance with the formula: [100×(1−(B/A))] (%). In this calculation, a method of determining the reference diameter should be needed. An example of this calculation method is disclosed in Japanese Unexamined Patent Application Publication No. 5-264232, which comprises the steps of estimating the reference diameter of a blood vessel from an angiographic image taken by a conventional two-dimensional X-ray imaging system, and calculating the stenosis ratio therefrom.
However, since a tubular structure such as a blood vessel has a complex shape because of the generally complex three-dimensional complex path, it is difficult to identify the position or the state of a disease such as stenoses and lumps even when observing a pseudo-three-dimensional displayed image (such as a volume-rendering image). When observing a two-dimensional image on an arbitrary sectional surface, on the other hand, it is very difficult to accurately set a sectional position, and this has imposed a burden on the operator, such as a physician.
These circumstances will be described in detail. The conventional three-dimensional observation method of a tubular structure such as a blood vessel or the large intestine involved various unsolved problems, as described below.
Firstly, when using a curved surface reformatted image, there is a problem in that it is difficult to readily grasp the position of a point in the curved surface reformatted image three-dimensionally and toward what direction it is directed.
Secondly, in the case of the method of displaying a sectional plane passing through the viewpoint position in a so-called “flythrough” display, information about the position in the blood vessel of a sectional plane at a position capable of being observed on the flythrough screen is unavailable.
Since the center line and the contour of the blood vessel are represented by many control points, manual editing takes additional time and labor.
Vascular diseases include aneurysm in which the blood vessel suffers from lumps. When the maximum diameter exceeds, for example, 5 mm, or the secular change in the maximum diameter exceeds, for example, 3 mm/year, the aneurysm may burst. It is generally believed that the patient should receive surgery. In the present circumstances, however, the maximum diameter of the aneurysm is observed and measured by use of an axial image. It is therefore difficult to grasp the three-dimensional shape or the secular change in diameter of the aneurysm, and the results are thus largely dependent upon the diagnostic ability and experience of the physician.